Semiconductors, or computer chips, are found in virtually every electrical product manufactured today. Chips are used not only in very sophisticated industrial and commercial electronic equipment, but also in many household and consumer items such as televisions, clothes washers and dryers, radios, and telephones. As products become smaller but more functional, there is a need to include more chips in the smaller products to perform the functionality. The reduction in size of cellular telephones is one example of how more and more capabilities are incorporated into smaller and smaller electronic products.
As the demand for semiconductor devices with low-cost, high performance, increased miniaturization, and greater packaging densities has increased, Multi-Chip Module (MCM) structures have been developed to meet the demand. MCM structures have a number of dies and other semiconductor components mounted within a single semiconductor package. The number of dies and other components can be mounted in a vertical manner, a lateral manner, or combinations thereof.
One such approach is to stack one die on top of another and then enclose the stack of dies in one package. The final package for a semiconductor with stacked dies is much smaller than would result if the dies were each packaged separately. In addition to providing a smaller size, stacked-die packages offer a number of advantages that relate to the manufacturing of the package, such as ease of handling and assembly.
In a stacked-die arrangement, the dies are wire-bonded sequentially, typically with automated wire-bonding equipment employing well-known thermal compression or ultrasonic wire-bonding techniques. During the wire-bonding process, the head of a wire-bonding apparatus applies a downward pressure on a conductive wire held in contact with a wire-bonding pad on the die to weld, or bond, the wire to the bonding pad on the die.
In many cases, stacked-die semiconductors can be fabricated faster and more cheaply than several semiconductors, each having a single die, which perform the same functions. A stacked-die approach is advantageous because of the increase in circuit density achieved.
Despite efforts to overcome problems resulting in lower yields of semiconductor packages with stacked dies, problems still exist. In particular, dies within the stack fail prematurely. Additionally, at least one die often overlies a plurality of other active or passive components, making designing of such semiconductor packages more difficult. Furthermore, the layout of bonding pads on the substrate is difficult, resulting in bonding wires of various lengths being used. The placement, as well as the parasitic inductance and parasitic capacitance of various-length bonding wires, needs to be taken into account during the design of the semiconductor package.
An upper die can crack during wire-bonding of the upper die due to lack of vertical support if the upper die overhangs the next lower die in the stack of dies. Consequently, smaller dies usually are placed on top of larger dies in semiconductor packages having stacked dies. Heat dissipation in semiconductor packages having multiple dies is a problem. The more dies that are placed in a semiconductor package, the greater the problem with heat dissipation.
The presence of multiple dies in a semiconductor package causes problems with the flow of the molding compound used to encapsulate the dies in forming the semiconductor package. There is also a problem with designing suitable electrical ground paths to the dies in a stacked-die semiconductor package.
When multiple dies are stacked in a semiconductor package, more adhesive material is used in the semiconductor package. Adhesive materials have the tendency to absorb moisture, which can have adverse effects on the dies, reducing the reliability and useful life of the semiconductor package. Furthermore, there is a thermal mismatch between the materials used to position the stacked dies in the semiconductor package and the substrate. The resulting relative motion of the dies with respect to the substrate has an adverse effect on the reliability of solder balls used to connect the dies to the substrate.
A need exists for a stacked-die arrangement which addresses the problems previously described, yet is efficient and cost-effective to manufacture. In addition, a need exists for a stacked-die arrangement which satisfies increased reliability test requirements.